Target designators, usually employing lasers, have in the past made use of a so-called “doublet pulse”, a pulse burst containing two pulses, in which the inter-pulse spacing is varied to provide a code which can be recognized by incoming missiles, guided shells or other ordinance devices. Doublet pulse laser target designators suffer from the disadvantage of requiring almost twice the laser energy per doublet pair,in order to provide range performance equal to a single pulse system.
In a recent development, a large portion of the extra energy requirement is compensated for by the superior efficiency provided by multi-cavity single-pump lasers or single-pump multi-laser systems in which sequentially activated Q-switches are provided in the laser-cavities. In these lasers a single pumping pulse provides the energy for the multiple pulse out-put. Typical efficiency increases measured in the laboratory and during field tests show that doublet pulses generated in this manner contain 50% more energy than those from a single cavity, multi-pump laser or lasers for a given amount of flash lamp energy.
While the multiple cavity, single-pump lasers provide a substantial increase in range, a still further substantial improvement can be obtained by utilizing doublet pulse integration. While the subject invention will be described in terms of two pulses, it will be appreciated that pulse integration of any number of regularly spaced pulses will result in substantial range increases in terms of the range at which a missile carrying a seeker or optical tracking device will lock onto a target illuminated by the multiple pulses.
The purpose of the pulse integration is to time superimpose a first pulse and a second pulse in a doublet pair, such that the signal components add in-phase, while the noise components add in random phase. In the two pulse case, this results in an increase in signal-to-noise ratio (SNR) of at 2 at least 3 db over direct doublet detection, which corresponds to an increase in the lock-on range of approximately 18%.
If pulse integration is utilized, the seeker of the missile will lock on to the target at ranges which exceed that which would ordinarily be achievable by detection of the doublet pulse. Thus, the seeker is able to lock on to relatively weak signals due to the pulse integration technique. In the subject system there are two modes of operation, namely, the short range or doublet decode mode and the long range or extended range mode. In the extended range mode doublet pulse decoding is not utilized, and therefore, there is a certain amount of countermeasure susceptibility at distances which are at the fringe of system performance.
However, in the extended range mode, countermeasure effectiveness is minimized, since even if countermeasuring is employed, the missile will, nonetheless, approach the target. As the missile or guided shell approaches the target, the intensity of the received radiation increases. When this intensity increases above a predetermined threshold, a doublet decoder within the missile's seeker is activated and signals resulting from detected radiation are only gated to the guidance system of the missile if doublet pulses having a known predetermined inter-pulse spacing have been received. Thus, in this embodiment, the seeker system is switched from its long range or extended range mode to its short range or doublet decode mode when the detected signals reach a predetermined threshold. At this point, the seeker is hardened against countermeasuring.
In summary, in the doublet decode mode, each incoming doublet pair is decoded and if the pair has the appropriate inter-pulse spacing the outputs from the pulse integrators are sampled and transmitted to the missile's guidance system. Any signals not having the requisite pulse coding are inhibited from reaching the guidance system of the missile.
The system described, while operating in a long range—short range mode, also has two additional modes of operation. The first mode of operation leaves the pulse integration system in operation all the time, whereas, the second mode of operation disconnects the integration system when the missile is operated in the short range or doublet decode mode. In either case, range extension is achieved by the pulse integration.
Superposition of the first and second pulses is accomplished, in one embodiment, by a recirculating delay line and summation network combination. In another embodiment, the delay line is replaced by a lighter and more versatile device, called a serial analog delay or SAD.
Assuming that the seeker utilizes the conventional quad cell detector, the output from each of these detectors is amplified and mixed in three channels such that if the quad cells are designated A, B, C, and D, then the outputs of the amplifiers are applied to processors which perform the following functions: (A+B)−(C+D); (A+B)−(B+C); and A+B+C+D. This provides three channels of information from the quad cell detector. The first two of these channels constitute the up-down channel and the right-left channels respectively which provide the directional signals for the missile's guidance system.
In one embodiment, the outputs of these processors are applied to respective recirculating delay units each of which have a feedback circuit to a summing network which adds the output of the recirculating delay unit to the incoming signal. The recirculating delay is exactly equal to the expected inter-pulse spacing such that the two;pulses coherently add in the summing network. The output of each recirculating delay unit is applied through a gate to either the up-down or right-left signal channels of the guidance system of the missile.
The third channel is utilized to detect when the missile is within a predetermined short range of the target. This is accomplished by applying the output of this channel to the same type of recirculating delay unit described for the first two channels. The output of this recirculating delay unit is applied to a high level trigger, which, in essence, activates a doublet decoder when the level of the radiation at the seeker reaches a predetermined level indicating that the missile is within a predetermined short range of the target.
In order to provide a signal to the doublet decoder, the third channel is coupled to the doublet decoder. The output of the doublet decoder is applied to a switching or gating system such that the switching or gating system generates a pulse which activates the gates in the first two channels in the presence of an appropriate pulse doublet to sample the quad cell output. When the system is in its short range or doublet decode mode, assuming that the appropriate signals are available at the quad cell, the doublet decoder decodes the fact that the pulses have the requisite interpulse spacing and applies a gating pulse to the gates of the first two channels. The delay throughout the doublet decoder/switching system corresponds to the expected inter-pulse spacing. Thus, if pulses of the appropriate inter-pulse spacing impinge on the quad cell their integrated values will be sampled and transmitted to the guidance system of the missile or ordinance device. Countermeasure signals are rejected by this system and do not affect the guidance system of the missile or ordinance device.
Alternatively, the pulse integration system may be completely taken out of the loop once the doublet mode threshold has been reached. This may have some advantages in close range situations where the signal-to-noise ratio is sufficiently high. However, it will be appreciated that by utilization of the recirculating delay units in the first two channels, an even higher signal-to-noise ratio can be obtained in these channels, with a consequent reduction in the false alarm rate.
It will also be appreciated that the system described includes “preprocessing” the signals from the quad cell detector prior to recirculation. If a recirculating delay line channel is provided for each quad cell detector, the processing for guidance purposes may be accomplished after the pulse integration in a “postprocessing” step.
It is therefore an object of this invention to provide an improved target designating system utilizing pulse integration for extending the range of the system;
It is another object of this invention to provide a target desiganting system which operates in a long range and a short range mode, in which at least the long range mode includes utilization of pulse integration for range extension;
It is another object of this invention to provide a method and apparatus for increasing the signal-to-noise ratio in seekers utilized with multiple pulse target designating systems.
These and other objects of the invention will be better understood in connection with the appended drawings in which: